1. Field of the Invention
This invention relates to a capping apparatus for capping containers, such as bottles, and, in particular, to a constant-torque capping apparatus capable of capping a container at a predetermined tightening torque accurately at all times. This invention is related to U.S. Pat. No. 4,535,583 issued Aug. 20, 1985, entitled "Rotary Type Capping Apparatus".
2. Description of the Prior Art
Typically, a prior art capping apparatus includes a turn table having a plurality of container holders disposed along the periphery of the turn table and a plurality of capping heads which are each provided corresponding in position to the container holders and driven to move along a circular path together with the turn table. Each of the capping heads has a cap holder which releasably holds a cap at its bottom and which is driven to rotate so as to have the cap screwed onto the mouth of the container held by the corresponding container holder on the turn table. In such a prior art capping apparatus, a sun gear is commonly provided as fixed in position and coaxial with a rotary shaft of the turn table and a plurality of pinions are provided in mesh with and disposed around the sun gear. Each of the pinions is fixedly provided on a driving shaft which is operatively connected to the corresponding cap holder so that the cap holder may be driven to rotate when the corresponding pinion moves around the sun gear in mesh therewith, thereby causing the cap held by the cap holder to be screwed onto the mouth of the corresponding container. In this prior art structure, a clutch is typically provided in a power transmitting system between the pinion and the cap holder and a slippage is induced in the clutch when the cap tightening force has reached a predetermined value.
However, in such a prior art capping apparatus, since the rotation of each pinion around its own axis depends on the rotation of the turn table, a torque for screwing a cap onto a container is directly determined by the rotation of the turn table. As a result, if the rotation of the turn table varies for some reason, the screwing or tightening torque also varies accordingly. This has been found to be extremely disadvantageous because the rotational speed of the turn table is sometimes desired to be set at different levels to accomodate other processing stations in the same container handling line, such as a filling station where desired contents are filled in the containers and a labelling station where labels are glued onto the containers. Moreover, even if the capping apparatus itself is operated at constant speed, the magnitude of inertia torque applied to the cap at the final stage of the capping operation tends to fluctuate for various reasons so that there has been encountered a difficulty in maintaining the cap tightening torque at a constant value with high accuracy.